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        <td class="header">&nbsp;Oil Spill Detection</td>


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<h3>Oil Spill Detection</h3>&nbsp;&nbsp;&nbsp;The operator detects dark spot&nbsp;such as oil spill&nbsp;on sea surface
from SAR imagery.<h4>Major Processing Steps</h4>&nbsp;&nbsp;&nbsp;The oil spill detection operation consists of the
following four operations:<br>
<ol>
    <li><span style="font-weight: bold;">Pre-processing:</span> Calibration and speckle filtering are applied to source
        image in this step.
    </li>
    <li><span style="font-weight: bold;">Land-sea masking:</span> Land-sea mask is created in this step to ensure that
        detection is focused only on area of interest.
    </li>
    <li><span style="font-weight: bold;">Dark spot detection:</span> Dark spots are detected in this step with an
        adaptive thresholding method.
    </li>
    <li><span style="font-weight: bold;">Clustering and discrimination:</span>
        Pixels detected as part of the dark spot are clustered and then
        eliminated based on the dimension of the cluster and user selected
        minimum cluster size.
    </li>
</ol>
&nbsp;&nbsp;&nbsp;For details of calibration and speckle filtering operations, the readers are referred to the <a
        href="./CalibrationOp.html">Calibration</a> operator and the <a href="SpeckleFilterOp.html">Speckle Filter</a>
operator.
&nbsp;Here it is assumed that the calibration and speckle filtering have
been performed before applying the oil spill detection operator.
<br>
&nbsp;&nbsp;&nbsp;For details of land-sea mask generation, the readers are&nbsp;referred to the <a
        href="CreateLandMask.html">Create Land Mask</a> operator.
<br>
<h4>Adaptive Threshold Algorithm</h4>&nbsp;&nbsp;&nbsp;The dark spots are detected using an adaptive thresholding
method.<br>
<ol>
    <li>First the local mean backscatter level is estimated using
        pixels&nbsp;in a large window.
    </li>
    <li>Then&nbsp;the detecting threshold is set
        k decibel&nbsp;below the estimated local mean backscatter level. Pixels
        within the window with values lower than the threshold are detected as
        dark spot. k is a user selected parameter (see parameter Threshold
        Shift below).
    </li>
    <li>Shift the window to next window position and repeat step 1 and 2.</li>
</ol>
<h4>Discrimination</h4>
<ol>
    <li>First the contiguous detected pixels are clustered into a single cluster.</li>
    <li>Then clusters with their sizes smaller than user selected Minimum Cluster Size&nbsp;are eliminated.</li>
</ol>
<h4>Visualize Detected Oil Spill</h4>

<p>The oil spill detection bit
    mask is output as a separated band. To&nbsp;view the oil spill detection
    results, the following steps should be followed:<br></p>
<ol>
    <li>Bring up the image.</li>
    <li>Go to&nbsp;<a href="../visat/LayerManagerWindow.html">Layer Manager</a> and add the oil spill bit mask band as a
        layer.
    </li>
</ol>
<h4>Parameters Used</h4>&nbsp;&nbsp;&nbsp;For dark spot detection, the following parameters are used (see figure 1):
<ol>
    <li>Source Bands: All bands (real or virtual) of the source product.
        User can select one or more bands for producing multi-looked images. If
        no bands are selected, then by default all bands are selected.
    </li>
    <li>Background Window Size: The window size in pixels for computing local mean backscatter level.</li>
    <li>Threshold Shift (dB): The detecting threshold is lower than the local mean backscatter level by this amount.
    </li>
</ol>
<img style="width: 600px; height: 500px;" alt="" src="images/oil_spill_detection_dlg.jpg">&nbsp;<br><br>&nbsp;
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;
&nbsp; &nbsp; &nbsp;Figure 1. Oil Spill Detection Operator dialog box.
<br><br><br>&nbsp;&nbsp;&nbsp;For clustering and discrimination, the following parameters are used (see Figure 2):<br>
<ol>
    <li>Minimum
        Cluster Size: The minimum&nbsp;cluster size in square kilometer.
        Cluster with size smaller than this size is eliminated.
    </li>
</ol>
<img style="width: 600px; height: 500px;" alt="" src="images/oil_spill_clustering_dlg.jpg"><br><br>&nbsp;
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;
&nbsp; &nbsp; &nbsp;Figure 2. Oil Spill Clustering operator dialog box.
<br><br>

<p style="font-style: italic;">Reference:</p>

<p> [1] A. S. Solberg, C. Brekke and R. Solberg, "Algorithms
    for oil spill detection in Radarsat and ENVISAT&nbsp;SAR images",
    Geoscience and Remote Sensing Symposium, 2004. IGARSS '04. Proceedings.
    2004 IEEE International,&nbsp;20-24 Sept. 2004, page 4909-4912, vol.7.</p>

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